My kids are learning from Beast Academy (in homeschool) and it's full of neat and useful ways to play with numbers.
Perhaps my favorite so far is a way to quickly determine if any number is divisible by 9, 6, and/or 3.
Say, this number:
(The photo below has my daughter looking at Latin, not math, but my husband's reaction is pretty much how I feel when I discover my kids have learned a shortcut I don't know. ;) )